Expandable linear power amplifier systems are often used in radio frequency communication systems, such as multicarrier cellular systems or other wireless communication systems. Expandable linear power amplifier systems typically include a number of linear power amplifiers operating in parallel and output signals are combined, as known in the art, into a single high power signal before transmission.
Such linear power amplifiers may employ feedforward error correction circuits, such as dual loop feedforward error correction circuits. Accordingly, improved linearity may be achieved with such linear power amplifiers. In such a design, the main amplifiers of a linear power amplifier may be coupled to one, two or more feedforward error correction loops. The main amplifier circuit may include a plurality of power amplifiers that are operating in a parallel fashion. The main amplifier circuits are typically larger and draw more current than the associated feedforward loops. Although improved linearity may be provided by, for example, two feedforward loops coupled to a main amplifier circuit, each loop adds delay that need to be corrected using low loss delay lines in main path. Also the feedforward linear power amplifier technique uses a pilot signal to monitor the error cancellation loop, which forms a closed loop system control. Such feedforward linear power amplifiers typically have relatively complicated feedforward circuitry and require delay circuit and other gain and phase adjustments.
One example of the operation of a feedforward amplifier may be found, for example, in U.S. Pat. No. 5,831,479, having inventor Leffel et al., issued Nov. 3, 1998, owned by instant assignee and incorporated herein by reference.
Another technique to linearize main amplifier output includes the technique of predistortion. FIG. 1 illustrates a conventional predistortion linear power amplifier 100 that uses serial intermodulation distortion (IMD) cancellation. The predistortion linear power amplifier 100 includes a predistortor 102. A signal splitter 104, power amplifiers 106 and 108 and a combiner 110 form the main power amplifier. The power amplifiers 106 and 108 are identical. The predistortor 102 as known in the art is used to generate, for example, a third order intermodulation distortion signal which is used to cancel the intermodulation distortion generated by the main amplifier due to the nonlinearity. The predistortor 102 and the main power amplifier are coupled in series. The predistortor 102 introduces a gain loss for the carrier signal so more amplification stages are required to compensate for the loss. For example, the predistortor 102 generates, for example, intermodulation distortion which is 180.degree. out of phase with the intermodulation distortion generated by the main amplifier. The intermodulation distortion is effectively added to the carrier signal and subsequently split into two paths by signal splitter 104. The power amplifiers 106 and 108 amplify the intermodulation distortion as well as the carrier signal. Due to the nonlinearity of the power amplifiers 106 and 108, the output signal from the main amplifier includes carrier signal, intermodulation distortion generated by the predistortor and the intermodulation distortion generated by the power amplifiers 106 and 108. Since the intermodulation distortion generated by predistortor has same magnitude but 180.degree. out of phase compared to the distortion generated by the main amplifier. The total intermodulation distortion of the output signal is significantly reduced. The combiner 110 (e.g., quadrature coupler) then combines the split carrier signal. This arrangement is typically an open loop arrangement.
Consequently, there exists a need for a linear power amplifier that utilizes predistortion that does not attenuate a main carrier signal to such a degree as a conventional predistortion linear power amplifiers.